Conservation of the vaquita Phocoena sinus - The Department of ...

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184 L. Rojas-Bracho, R. R. Reeves and A. Jaramillo-Legorreta Fig. 2. Northern and upper Gulf of California, showing the approximate distribution of vaquitas. The circles represent vaquita sightings and acoustic encounters. The star represents the position of the Rocas Consag. The Biosphere Reserve is delimited to the south by an imaginary line connecting San Felipe to Puerto Peñasco; therefore part of the vaquita distribution is outside of the reservé. Bay. This area is generally referred to as ‘The Hot Spot’ for vaquitas, as at least one group has been detected every time the detection equipment has been deployed there. Norris was told by the captain of a shrimp trawler in 1958 that he had caught vaquitas in nets set for totoaba Totoaba macdonaldi (a large fish of the croaker family Sciaenidae that is endemic to the Gulf of California) ‘in the estuary of the Río Colorado at the head of the gulf’ (Norris & Prescott, 1961). He also described having taken them at the entrance to a small lagoon north of San Felipe, where they became trapped in the nets at low tide. Similar © 2006 The Authors. Issue compilation © 2006 Mammal Society, Mammal Review, 36, 179–216
© 2006 The Authors. Issue compilation © 2006 Mammal Society, Conservation of the vaquita 185 reports were given to the authors by University of Baja California researchers and enforcement officers from San Felipe with long experience in the northern Gulf: N. Castro and V. Zazueta in 1997 and 1999. This information suggests that vaquitas can occur in shallow, tidally influenced estuarine waters. It is important, however, to treat such observations with caution because of the possibility that vaquitas have been confused with bottlenose dolphins Tursiops truncatus, which are, according to Silber (1990), ‘consistently seen in more shallow water’ than vaquitas (also see Silber et al., 1994). Abundance Historica The vaquita presents an essentially insurmountable challenge in trying to produce a plausible estimate of historical abundance. Villa-Ramírez (1976) concluded from talking to fishermen in the northern Gulf that vaquitas were formerly ‘very abundant’, but he also admitted that the fishermen had difficulty both detecting vaquitas and distinguishing them from other small cetaceans, particularly small bottlenose dolphins. Vaquitas are notoriously difficult to detect and observe, so the usual caution concerning the reliability of qualitative historical observations of animal abundance by non-specialists applies with special force in this case. Even if the memories of fishermen were a reliable source of information on past abundance, few with relevant knowledge would be alive today and therefore available for interrogation. Modern nylon gill nets were being used in the northern Gulf to catch totoaba by the early 1940s (Cisneros-Mata, Montemayor-López & Román-Rodríguez, 1995), so it must be assumed that very high incidental mortality of vaquitas was occurring (see later), and therefore that the vaquita population was already declining by then. No polymorphism has been found in the mitochondrial DNA control region (Rosel & Rojas-Bracho, 1999) or in the DQB locus of the Major Histocompatibility Complex (MHC) (Munguía, 2002) of 43 and 25 vaquita specimens, respectively. These findings are consistent with the hypothesis that the evolutionary history of the species included a bottleneck or founder event, possibly at species inception, followed by a long period (perhaps some 10 000 years) of persistence at a small effective population size. Judging by the results of simulations of plausible population dynamics, vaquitas have probably always been rare, and their extreme loss of genomic variability likely occurred over evolutionary time (Taylor & Rojas-Bracho, 1999; Munguía et al., 2003a,b). Recent Qualitative references to rarity and low numbers are scattered throughout the vaquita literature from the time of the species’ description (Norris & McFarland, 1958). Even with stateof-the-art survey tools and methods available starting in the 1980s, the vaquita has presented an extreme challenge for abundance estimation because of its murky habitat and elusive surfacing behaviour (Silber & Norris, 1991; Barlow et al., 1993; Fig. 3). Among the characteristics that complicate surveys for vaquitas, even under the best of weather conditions, are: • The animal’s small size, with a triangular dorsal fin up to about 15 cm high; • They spend most of their time under water and are visible only for about 3 s at a time, at surfacing intervals that average between 1 and 1.5 min (Silber, Newcomer & Barros, 1988); • When surfacing, they rarely splash or jump; • Group size averages only about two individuals; and • They generally avoid boats and ships. Encounter rates obtained in aerial surveys ranged from 1.8 to 7.2 vaquitas per 1000 km (Silber, 1990; Silber & Norris, 1991; Barlow et al., 1993). These are low figures when com- Mammal Review, 36, 179–216
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